Recycle of monoolefines to a hydrocarbon pyrolysis process



Sept. 6, 1966 w. KRNlG ET AL 3,271,297

RECYCLE OF MONOOLEFINES TO A HYDROCARBON PYROLYSIS PROCESS Filed Dec. 11, 1961 HYDROGENATION UNIT 7 PYROLYSIS 3 DISTILLATION [COLUMN 1 4 DlSTlLLATiON IN VEN TORS I WALTER KRd/v/e. HANS JOACH/M MULLER mm HE/NZ VO'PEL.

TTORNE 5 Claims. (a. 208-67) This invention relates to a cracking process and more particularly to the removal of aromatic compounds from cracked gasoline. In the production of gaseous olefines by the pyrolysis of liquid hydrocarbons a cracked gasoline is obtained which has a high content of unsaturated compounds, such as olefines, but which also contains not insignificant quantities of aromatic compounds (benzene hydrocarbons). Whereas the benzene hydrocarbons can be utilised, the fractions of the cracked gasoline which are free from aromatic substances frequently cannot. Consequently, attempts have been made to return the fractions of the cracked gasoline which are free from aromatic substances to the cracking process. However, considerable difiiculties arise during the heating of these fractions, in that gum-like deposits are formed in the heating assemblies.

It is an object of the present invention to overcome the aforementioned difliculties. It is a further object to pro- Vide a simple method for the recovery of aromatic compounds from cracked gasoline. Further objects will appear hereinafter. It has now been found that the aforementioned objects can be attained if in the liquid hydrocarbons (cracked gasoline) obtained in the pyrolysis in a first step practically only the diolefines are hydrogenated but the monoolefines and aromatic substances are substantially not hydrogenated and if the thus hydrogenated hydrocarbons are then substantially freed from aromatic substances and if the remaining fractions freed from aromatic compounds are then returned to the pyrolysis process.

The hydrogenation of the cracked gasoline can be carried out in known manner. It is preferably carried out in the liquid phase. Desirably the liquid is provided in the form of a film. Precious metal catalysts and more especially palladium catalysts which have preferably been on supports, are suitable catalysts for this purpose. It is, however, also possible to use nickel catalysts, preferably on a support. In the case of the precious metal catalysts, more especially the palladium catalysts, it is preferable to work at a low temperature, for example at a temperature below 100 C. and preferably at a temperature below 50 C. The hydrogenation of the cracked gasoline may be carried out by maintaining the first part of the reaction chamber at a temperature below 50 C., the temperature in the other parts of the hydrogenation chamber rising to, for example, 75 C. or 100 C. With nickel catalysts, the entire reaction chamber may be maintained above 50 C. and the temperature may rise to 200 C. Hydrogenation pressures of from 20 to 80 atmospheres and preferably from 30 to 50 atm., are employed. Heating of the hydrogenation products does not present any ditficulties when the cracked gasoline fractions are hydrogenated in this way. No deposits are formed in the heating system and the process can be operated continuously without any interruption.

The cracked gasoline fraction, which boils below the boiling point of benzene, i.e., below 75 C., and which has been obtained after the hydrogenation is very suitable as feed for pyrolysis process and can be returned thereto without difliculties. However, the fractions of the cracked nited States Patent Patented Sept. 6, 1966 gasoline which are of higher boiling point and which are substantially free from aromatic substances may also be returned to the pyrolysis process. These fractions may be obtained, e.g., in the following manner: The fractions separated by distillation from the cracked gasoline which has been hydrogenated as previously described and which contain aromatic substances, as well as benzene and homologues thereof (where it is desired to recover these aromatic substances) are hydrogenated in the gas phase in known manner under such conditions that the olefines are practically completely hydrogenated but the aromatic substances are almost completely retained. For this purpose, it is advantageous to use the known hydrogenation methods in the gaseous phase, preferably over fixed oxide or sulfide catalysts, for example cobalt molybdate on aluminum oxide, molybdic acid on aluminum oxide etc. The process is generally carried out at temperatures between 250 and 400 C. and pressures between 10 and 60 atmospheres, and the rate of feed of reaction mixture is between 0.5 and 3 kg. per litre of catalyst per hour. The hydrogenation may, if desired, be carried out in the liquid phase. In this second hydrogenation, the reaction conditions are so adjusted that hydrogenation is carried out until a bromine number of practically zero is obtained and aromatic products are obtained. If the aromatic sub stances are then isolated in known manner from the hydrogenated fraction there is obtained a rafiinate which is practically free from aromatic substances and which can be returned to the cracking process, either by itself or together with the aforementioned fraction boiling below C. and which has been obtained from the first hydrogenation stage.

The invention is represented in the accompanying flow diagram. Liquid hydrocarbons are introduced through line 1 into the pyrolysis vessel A. The reaction products of this pyrolysis are introduced through line 2 into the distillation column B. At the top of this distillation column gaseous products are removed through line 3 while through line 4 a residue is removed. Through line 5 cracked gasoline is introduced into a hydrogenation unit C wherein practically only the diolefins are hydrogenated while mono-olefins and aromatic compounds are not hydrogenated. At the bottom of this column the hydrogenation product is taken out via a line 6 and is introduced into the distillation column D. At the bottom of this distillation column the compounds boiling above about 75 C. are taken out including the aromatic fraction while at the top of this distillation column D products boiling below 75 C. which are practically free from aromatic compounds are recycled through line 7 to the pyrolysis.

Example A cracked gasoline boiling between 30 and 200 C. was obtained in a quantity of 35% by the pyrolysis of a distillate gasoline of a Middle East crude oil. The cracked gasoline was hydrogenated in the liquid phase, with the liquid in the form of a film, over a catalyst which contained 1% of palladium on sintered aluminum oxide and which had an intrinsic surface of 3 m? per gram. The temperature was 25 C. at the inlet to the reactor, 50 C. in the middle of the reactor and 70 C. at the outlet end of the reactor. The hydrogen pressure was 30 atmospheres. The hydrogenation product obtained, which was practically free from diolefines, was split by distillation into a fraction boiling up to a temperature of 75 C. and a fraction boiling above this temperature. The fraction boiling above 75 C. was hydrogenated in the gas phase at a hydrogen pressure of 50 atm. over a cobalt-molybdate catalyst at 320 C. The hydrogenation product had a bromine number of 50 mg. per cc. and the aromatic substances remained practically unchanged as compared with the starting material. The hydrogenation product was extracted in known manner with a water-containing diethylene glycol, the aromatic substances being extracted leaving the non-aromatic substances. The nonarornatic traction was combined with the fractions boiling up to 75 C. which had been obtained from the first hydrogenation stage and returned to the pyrolysis process. At this stage, the operation was carried out at a temperature of 740 C. in the presence of steam in an amount of 50 parts by weight per 100 parts by weight of the material being used. There were thus obtained 65 parts by weight of gaseous hydrocarbons containing not more than 4 carbon atoms and 35% of liquid hydrocarbons, predominantly in the gasoline boiling range. The yields of gaseous olefines were 20% of ethylene, 16% of propylene, 14% of C -olefines-i-diolefines. The cracked gasoline contained 15% of benzene, 8% of toluene and 4% of higher aromatic substances.

We claim:

1. In a process for the pyrolysis of liquid hydrocarbons wherein a feed stock is subjected to pyrolysis to produce a cracked gasoline containing monoolefins and diolefins and aromatic compounds, the improvement which comprises subjecting the cracked gasoline to a hydrogenation treatment to convert diolefins of the cracked gasoline tomonoolefins without substantial hydrogenation of monoolefins, separating monoolefins from the product of hydrogenation to provide to a monoolefin fraction and an aromatic fraction, and introducing said monoolefin fraction into the pyrolysis.

References Cited by the Examiner UNITED STATES PATENTS 2,106,735 2/1938 Gwynn 20857 2,138,881 12/1938 Pyzel 20857 2,376,086 5/1945 Reid 20857 2,382,910 8/1945 Pinkston 20867 2,431,243 11/ 1947 Greensfelder et al 20896 2,432,537 12/1947 Newton 20874 2,717,861 9/1955 Baumgarten et al. 208144 2,953,612 9/1960 Haxton et al 20857 3,075,917 1/ 1963 Kronig et al 208144 3,108,947 10/1963 Stijntjes 260677 DELBERT E. GANTZ, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner.

H. LEVINE, Assistant Examiner. 

1. IN A PROCESS FOR THE PYROLYSIS OF LIQUID HYDROCARBONS WHEREIN A FEED STOCK IS SUBJECTED TO PYROLYSIS TO PRODUCE A CRACKED GASOLINE CONTAINING MONOOLEFINS AND DIOLEFINS AND AROMATIC COMPOUNDS, THE IMPROVEMENT WHICH COMPRISES SUBJECTING THE CRACKED GASOLINE TO A HYDROGENATION TREATMENT TO CONVERT DIOLEFINS OF THE CRACKED GASOLINE TO MONOOLEFINS WITHOUT SUBSTANTIAL HYDROGENATION OF MONOOLEFINS, SEPARATING MONOOLEFINS FROM THE PRODUCT OF HYDROGENATION TO PROVIDE TO A MOOLEFIN FRACTION AND AN AROMATIC FRACTION, AND INTRODUCING SAID MONOOLEFIN FRACTION INTO THE PYROLYSIS. 